The present invention relates to an etching method whereby a target film, such as a metal alloy film, formed on a wafer is etched by using a plasma in the process of manufacturing a semiconductor device and to an etching apparatus used in the etching method.
A method of generating a plasma by utilizing RF discharge has found a wide range of applications in such fields as plasma etching for micro-fabrication and plasma CVD for thin-film formation, each conducted in a semiconductor process. In particular, the field of plasma-assisted dry etching has required the establishment of a dry-etching technique which promises a high-precision and stable etching process with improved reproducibility, productivity, and yield.
In the field of dry etching performed with respect to a target film such as a metal alloy film, a method of reactive ion etching (RIE) has been used predominantly.
A description will now be given to a conventional plasma-etching apparatus.
FIG. 8 shows the schematic structure of the conventional RIE plasma-etching apparatus. A chamber 1 maintained under vacuum is internally provided with a sample stage 3 for carrying a wafer 2 as a sample to be etched. For example, RF power of 13.56 MHz is applied from an RF power source 4 to the sample stage 3, which also serves as a lower electrode. The chamber 1 is formed with a gas inlet 5 for introducing reactive gas into the chamber 1 and with a gas outlet 6 for exhausting the reactive gas from the chamber 1. On the sample stage 3, there is provided a focus ring (equalizer ring) 7 with a height of 10 to 20 mm, which is positioned around the wafer placed on the sample stage 3 to equalize the distribution of radicals composed of reactive gas and directed to a target film on the wafer 2.
In the conventional process of etching the target film such as a metal alloy film, optimum etching conditions under which the uniformity of the etching rate is .+-.5% are predetermined so that the target film is etched under the optimum etching conditions.
To optimize the etching conditions, there have been adopted a method of replacing the focus ring 7 with another of a different height and a method of controlling the direction in which the reactive gas blows into the chamber 1. Briefly, the level of the focus ring 7 and the direction of the blowing gas are varied during actual etching till the optimum level and direction that provide the optimum etching rate are achieved.
Table 1 shows an example of etching conditions for dry etching performed with respect to a metal alloy film.
TABLE 1 PARAMETER VALUE POWER 700 W (13.56 MHz) GAS SPECIES 100/100/20 sccm (BCl.sub.3 /Cl.sub.2 /N.sub.2) PRESSURE 150 mTorr
FIG. 9(a) shows the cross-sectional structure of a sample before dry etching is performed. FIG. 9(b) shows the cross-sectional view of the sample after dry etching was performed. As shown in FIG. 9(a), a BPSG film 11, a first TiN film 12, an Al-1% Cu film 13, and a second TiN film 14 are successively deposited on a semiconductor substrate 10 made of silicon. A resist pattern for forming a pattern with a 0.7 .mu.m line width is formed on the second TiN film 14. When dry etching is performed with respect to the second TiN film 14 and Al-1% Cu film 13 masked with the resist pattern 15 under the etching conditions shown in Table 1, a metal wire 16 with a 0.7 .mu.m line width is formed as shown in FIG. 9(b).
Table 2 shows the relationship between the number of processed wafers with a diameter of 8 inches and the etching rate.
TABLE 2 UNIFORMITY OF ETCHING RATE (%) WHEN PROCESS IS WHEN PROCESS IS STABLE UNSTABLE .sup. 1ST WAFER .+-.3.9 .+-.4.0 5TH WAFER .+-.4.8 .+-.8.0 10TH WAFER .+-.4.1 .+-.5.3 15TH WAFER .+-.4.6 .+-.10.9 20TH WAFER .+-.4.3 .+-.8.7 25TH WAFER .+-.4.6 .+-.4.5
As shown in Table 2, when the number of processed wafers is small and the etching process is stable, the uniformity of the etching rate remains within .+-.5% for each of 6 wafers selected from 25 wafers and examined for the etching rate. However, as the number of processed wafers increases and the conditions in the reaction chamber change, the etching process becomes unstable. When the etching process is unstable, the uniformity of the etching rate becomes .+-.5% or more, as indicated by the 5th, 15th, and 25th wafers, resulting in a reduced etching rate.
Although the foregoing data represents the relationship between the number of processed wafers with a diameter of 8 inches and the etching rate, the aforesaid tendency is more noticeable in the case of performing etching with respect to wafers with a diameter of 12 inches. In other words, the non-uniformity of the etching rate is more conspicuous with wafers larger in diameter.
However, even when the uniformity of the etching rate is degraded, the conventional etching method continues etching without changing etching conditions. It is not until the problem arises that the focus ring is replaced with another of a different height or the direction of the blowing reactive gas is changed after the pressure inside the reaction chamber is switched from the vacuum state to an atmospheric state.
As a result, the reliability of a semiconductor device becomes less stable and the yield lowers due to the degraded uniformity of the etching rate.